The present invention relates to telephone interface devices and, in particular, to interfaces designed to be inserted between a telephone input device and a telephone host system.
British Patent Application No. 984905.9 describes a system for diagnosing the specific connections for the handset to the telephone base unit by considering the appearance of DC voltage and dial tone when the handset is in the off-hook condition. The diagnoses are performed automatically.
This diagnostic routine and a similar system described in WO 97/36411 depend on dial tones which vary in amplitude with transmission distance and which differ from country to country in frequency content and amplitude modulation patterns. Thus, no one dial tone detection scheme can be used to diagnose the earphone connection terminals. Another disadvantage is that the base unit must be connected to the host telephone system, which in turn must be connected to an active telephone host system, complicating the replacement of a handset.
In an embodiment the present invention includes an interface device that (1) automatically diagnoses the type and terminal connections of the earphone and microphone transducers used in any handset or headset and (2) automatically connects any transducer set provided as a replacement handset or headset to emulate the original handset or headset. Subsequently, the user can connect, without further action, the replacement transducer set to the telephone base station. By applying this invention, the user achieves comparable performance in sensitivity, output impedance, and current drain for DC line supervision as the original handset or headset but with the added convenience of upgraded performance provided by the new transducer set arrangement.